CDMA offers the possibility of combining multiple discrete-time data streams into a total signal such that the original data streams can be recovered from this signal. Data streams can be recovered if the CDMA-coded signals are modified by different linear filters. The simplest possibility of data detection is called a matched filter. Better results are obtained with a RAKE receiver, as described, for example, by A. Salmasi, K. S. Gilhousen: “On the system design aspects of code division multiplex access (CDMA) applied to digital cellular and personal communications network,” Proc. IEEE Conf. Veh. Technol., St. Louis, Mo., USA, May 1991, pages 57-62. These detectors require only a knowledge of the code of the data stream to be detected. If the codes of the data streams not to be detected are also known, interference caused by foreign data streams (due to non-orthogonal codes) can be determined and eliminated. CDMA detection can be improved significantly by appropriate multiuser methods. In addition to the optimal maximum likelihood detector, described, for example, by W. van Atten: “Maximum likelihood receiver for multiple channel transmission systems,” IEEE Trans. Commun., vol. 24 (1976) 276-283, several suboptimal methods are known, such as those described by W. Sauer-Greff and R. A. Kennedy: “Suboptimal MLSE for distorted multiple-access channels using the M-algorithm,” Proc. Aachener Kolloquium Signal Theorie, Aachen, March 1994, 267-270; K. S. Schneider: “Detection of code division multiplexed signals,” IEEE Trans. Aerosp. Electron. Syst., vol. AES-15 (1979), 181-185; R. Lupas and S. Verdu: “Linear multi-user detectors for synchronous code division multiple-access channels,” IEEE Trans. Inform. Theory, vol. 35 (1989) 123-136; Z. Xie, R. T. Short and C. K. Rushforth: “A family of suboptimum detectors for coherent multi-user communications,” IEEE J Select. Areas Commun., vol. 8 (1990), 683-690; and A. Duel-Hallen: “Decorrelating decision-feedback multiuser detector for synchronous code-division multiple-access channels,” IEEE Trans. Commun., vol. 41(1993) 285-290.
If the pulse responses of the transmission channels of the coded data streams are known, interference caused by filtering can also be eliminated, as described by J. Salz: “Digital transmission over cross-coupled linear channels,” Bell Syst. Tech. J, vol. 64 (1985), 1147-1159; A. Duel-Hallen: Equalizers for multiple input/multiple output channels and PAM systems with cyclostationary input sequences,” IEEE J. Select. Areas Commun., vol. 10 (1992) 630-639; and M. L. Honig, P. Crespo and K. Steiglitz: “Suppression of near- and far-end crosstalk by linear pre- and post-filtering,” IEEE J. Select. Areas Commun., vol. 10 (1992), pages 614-629.
The greatest disadvantage of multiuser detectors is their high computational complexity. Even suboptimal methods often cannot be implemented because of the computational complexity. On the other hand, the RAKE receiver, which is simple in computing terms, offers greatly inferior detection results.